Macrophages (M0) and T cells produce a variety of soluble factors which modulate the induction of the antibody and cell-mediated immune responses. The biochemical relationship between the multiple factors, their mode-of-action, and conditions under which they are produced have been investigated. Interleukin 2 (IL 2), which augments plaque-forming cell (PFC) and cytotoxic T cell responses, has been shown to be T cell-derived although M0-dependent. The M0 requirement during the production of IL 2 can be replaced by either phorb 1 myristic aceptate or M0 culture supernatants. Interleukin 1 (Il 1) (LAF) has been shown to be 1) produced in vivo, 2) capable of replacing the requirement for M0 during the induction of primary T cell responses and 3) capable of synergizing with a late-acting T cell replacing factor (TRF) in the induction of the antibody response. TRF has been shown to be distinct from the principal component of IL 2 on the basis of differential sensitivity to pH 2.0 and partial separation by hydrophobic chromatography. Thus at least three distinct immunoenhancing factors (IL 1, IL 2, and TRF) have been partially delineated biochemically and functionally. Studies are currently in progress to attempt to elucidate the mechanism by which these factors function in concert to enhance B cell and T cell responses.